Polyimide resin is widely used for electric and electronic purposes due to its excellence in heat resistance, electric insulation reliability, chemical resistance, and mechanical properties. For example, polyimide resin is used as a substrate material and/or a surface protecting material for a flexible circuit board, an integrated circuit board, or the like. Alternatively, polyimide resin is used to provide a semiconductor device with an insulating film and/or a protective coating film, and also to provide a microcircuit with an interlayer insulating film and/or a protective film.
In particular, a cover lay film obtainable by applying an adhesive to a molded article such as a polyimide film or the like is used in a case where polyimide resin is used as a surface protecting material for a flexible circuit board. The cover lay film is generally bonded to the flexible circuit board by providing in advance, by punching or the like, an opening for a junction of the cover lay film with a terminal section or a component of a circuit, positioning the cover lay film and the flexible circuit board, and then carrying out thermo-compression bonding by hot pressing or the like with respect to the cover lay film and the flexible circuit board which have been positioned.
However, it is difficult to provide a fine opening for a thin cover lay film. Further, a cover lay film and a flexible circuit board are frequently manually positioned so as to be combined. This causes not only a deterioration in positional accuracy but also a deterioration in workability of the combining, and consequently causes an increase in cost.
Meanwhile, a solder resist, for example may be used as a surface protecting material for a circuit board. In particular, a solder resist having a photosensitive function is preferably used in a case where fine processing is required. A photosensitive resin composition which consists mainly of acid-modified epoxy acrylate, epoxy resin, or the like is used as such a photosensitive solder resist. However, the photosensitive solder resist is low in mechanical properties such as bending resistance and the like and large in cure shrinkage while being excellent in electric insulation reliability as an insulating material. Therefore, in a case where the solder resist is laminated on a circuit board such as a flexible circuit board or the like which is thin and rich in flexibility, the circuit board is largely warped. This makes it difficult to use the photosensitive solder resist for a flexible circuit board.
In recent years, an attempt has been made to protect, as below, confidential information which is included in a circuit pattern. The confidential information is protected by covering the circuit pattern by coloring black a cover lay film or a photosensitive solder resist, which serves as a surface protecting material for a circuit board, so as to provide the cover lay film or the photosensitive solder resist with coverability.
In particular, it is important for a photosensitive solder resist to have both (i) photosensitivity, flexibility, heat resistance, and plating resistance, and (ii) blackness and coverability. Against a background of this, various proposals have been made for allowing these properties to be apparent.
For example, a black photo solder resist which has sufficient blackness and is excellent in resolution has been proposed (for example, refer to Patent Literature 1).
Further, black photo solder resist ink which is highly photo-curable has been proposed (for example, refer to Patent Literature 2).